(1) Field of the Invention
The present invention relates to a resonant circuit inverter with a controllable operating point.
(2) Description of Related Art
Resonant circuit inverters are described, for example, in the textbook “Leistungselektronik” (Power Electronics) by Rainer Felderhoff, published by Carl Hanser Verlag, 2nd edition, ISBN 3-446-18993-9. The resonant circuit inverters described therein have, for example, a three-phase rectifier, a DC-link circuit and a single-phase inverter, with the output of the inverter forming the output of the resonant circuit inverter through which the current flows to the connected load.
Resonant circuit inverters are frequently used as frequency converters.
The document EP 0 617 503 A1 describes a method and an apparatus for controlling the operating point of a resonant circuit inverter. The resonant circuit inverter disclosed in this document is used for inductive heating of a heated material. The document describes the significance of the frequency of the current in the load circuit selected for inductive heating. The frequency of the output current is advantageously either slightly greater than the resonance frequency or slightly smaller than the resonance frequency, depending on the selection and/or availability of the electro-technical components. As also described in the document, the load circuit then assumes an inductive characteristic or a capacitive characteristic, which means that the output current is either lagging or leading the voltage at the output of the resonant circuit inverter. The operating point for operating the resonant circuit inverter described in the document is determined by the phase angle between the output current and the voltage at the output of the resonant circuit inverter.
In the aforedescribed inductive heating application, the parameter in the load circuit changes due to the heating, causing the resonance frequency to change. The phase angle between the output current and the voltage at the output of the resonant circuit inverter also changes, i.e., the operating point changes, which is undesirable.
It was an object of the solution described in the aforementioned document to propose a method and an apparatus for controlling the operating point of a series resonance circuit inverter, so that the predetermined operating range for the resonant circuit inverter can be identified and kept stable independent of parameter variations of the oscillating circuit elements.
The aforementioned document discloses a control of the phase angle, wherein the controller evaluates the difference between a nominal phase angle and an actual phase angle and depending on the evaluation adjusts the frequency of the output current by operating on the inverter. The phase angle may be, for example, the angle between the output current and the voltage at the output of the inverter.
The document also describes using the phase angle between other quantities, because using particularly the output current may sometimes be disadvantageous.
A resonance circuit inverter can not only be used to inductively heat materials. Materials may also be heated by conducting current through the materials and using their effective resistance, as is the case, for example, when heating silicon rods or thin silicon rods in the manufacture of polysilicon by chemical vapor deposition with the Siemens process.
As in inductive heating of materials, when heating silicon rods or thin silicon rods (also referred to as silicon seed rods), the growth of the silicon rods or thin silicon rods changes parameters in the oscillating circuit. The resonance frequency of the oscillating circuit then also changes with chemical vapor deposition, requiring an adjustment of the operating point commensurate with this change. The operating point can be adjusted in vapor deposition as well as in inductive heating by changing the phase angle between the output current and the voltage at the output of the resonant circuit inverter.
Double-commutations may occur in a resonant circuit inverter, in particular when using a single phase inverter with H-connection, at frequencies close to the resonance frequency, which causes in short succession directional changes in the output current. These multiple directional changes result in several zero crossings of the output current, making detection of a zero crossing for detecting the phase angle between the output current and the voltage at the output of the resonant circuit inverter at least difficult.
This is a starting point for the invention.